1. Field of the Invention
The present invention relates to a crucible assembly for deposition of an organic thin film or a conductive layer, and an inner plate used in the crucible assembly.
2. Description of Related Art
In general, physical vapor deposition (PVD), such as vacuum deposition, ion-plating, sputtering, and chemical vapor deposition (CVD) through a reaction of gases are well known as a methods of forming a thin film on a substrate. Among them, the vacuum deposition can be used to form a thin film such as an electrode or an organic film for an organic light emitting device.
Vacuum deposition is a technique for forming a thin film using an evaporation source provided at a lower portion of a vacuum chamber and a substrate for film formation provided at an upper portion of the vacuum chamber. Generally, in a thin film forming apparatus using vacuum deposition, the interior of a vacuum chamber is maintained under a predetermined vacuum atmosphere using a vacuum pump, and a deposition material, which is a thin film material, is then evaporated from an evaporation source located at a lower portion of the vacuum chamber. A substrate for film formation located at a predetermined distance from the evaporation source is positioned within the interior of the vacuum chamber. Therefore, the deposition material evaporated from a crucible travels to the substrate and is solidified on the substrate for film formation through consecutive processes including absorption, deposition, re-evaporation and the like, thereby forming a thin film.
An effusion cell using an induction heating method, which is called an indirect heating method, is frequently used as an evaporation source for the vacuum deposition. The effusion cell includes a crucible having a deposition material accommodated within an interior thereof and a heater wound about an outer circumferential surface of the crucible to electrically heat the deposition material. A top of the crucible is opened so that vapor can be expelled to the outside of the crucible when the deposition material accommodated within the crucible is evaporated. A cap member provided with an aperture having a predetermined size is typically formed at an upper portion of the crucible so as to prevent the deposition material from being splashed to the outside of the effusion cell when the deposition material is evaporated. The cap member also serves to assure a reproducible thickness of a film on the substrate for film formation.
However, in the effusion cell used in the aforementioned thin film forming apparatus, heat loss at the upper portion of the crucible is generated by the aperture in the cap member formed at the upper portion of the crucible. The deposition material that evaporates within the interior of the crucible and then travels to the substrate for film formation can condense around the aperture due to the rapid variation in temperatures between the interior and the exterior of the cap member. As a result, the aperture of the cap member can become blocked by the condensed deposition material. The blockage of an aperture in the cap member can cause the deposition rate in a deposition process to become unstable. What is therefore needed is a design for a crucible assembly that does not allow the aperture in the cap assembly to become blocked.